1. Field of the Invention
This invention relates to unitary, self-generating reference gas sensors, useful to monitor not only a SO.sub.2, CO.sub.2 or NO.sub.2 component, but also an O.sub.2 gas component of a monitored gas environment.
2. Description of the Prior Art
The requirements for monitoring and controlling stack gas pollutants have resulted in the development of solid electrolyte gas sensors having electrolyte compositions uniquely responsive to gases such as SO.sub.2, CO.sub.2 and NO.sub.2. These sensors are electrochemical concentration cells which sense the equilibrium of a gas species of interest and generate an EMF signal corresponding to the difference in partial pressure of the gas species across the solid electrolyte sensor. Typically, the solid state sensor includes an ion conductive solid electrolyte with electrodes disposed on its opposite surfaces. The stack gas, or monitored gas stream, contacts a sensing electrode, while the opposite electrode serves as a reference electrode which is contacted with a reference gas stream. Conventional solid electrolyte compositions require operating temperatures of between 200.degree. C. and 900.degree. C. to exhibit the desired ion conductivity to generate a suitable EMF signal.
In the past, a major problem with these devices was isolation of the monitored gas from the reference gas, to prevent unpredictable drift in the measurement signal. Hirayama et. al., in U.S. Pat. No. 4,377,460, solved this sealing problem by using a closed end, gas impermeable, mullite (3Al.sub.2 O.sub.3 .multidot.2SiO.sub.2) tube, which acts as an alkali ion conductive membrane at high temperatures. The mullite tube, like most ceramics, incorporates some alkali oxide impurities, such as K.sub.2 O, making it a K.sup.+ ionic conductor at high temperatures. This tube was used to separate the two gas streams and provide two identical alkali ion conductive half cells secured to opposite sides of the mullite.
The two, alkali ion conductive solid electrolyte discs used in each half cell of the Hirayama et al. design, to monitor SO.sub.2, CO.sub.2 or NO.sub.2, were made of K.sub.2 SO.sub.4, Na.sub.2 CO.sub.3, or NaNO.sub.3 respectively. A platinum electrode was attached to one side of each half cell electrode. In the case of a SO.sub.2 +O.sub.2 reference gas stream, this provided the cell assembly: EQU SO.sub.2 +O.sub.2 Reference Gas, Pt.vertline.K.sub.2 SO.sub.4 .vertline.Mullite.vertline.K.sub.2 SO.sub.4 .vertline.Pt,SO.sub.2 +O.sub.2 Flue Gas
Lin et al., in U.S. Pat. No. 4,427,525, taught a somewhat similar system, but used calcia stabilized zirconia as the tube membrane. This tube membrane is oxygen ion permeable. A separate cell which can be used to measure the partial pressure of O.sub.2 is disposed across this tube, removed a distance from the two half cells used to monitor SO.sub.2, CO.sub.2 or NO.sub.2, with common circuitry connecting all the cells, making it a dual gas sensing apparatus.
In an attempt to not only effectively seal monitored gas from the reference gas, but to also eliminate the effect of O.sub.2 on the EMF signal measurement of SO.sub.2, Lin et al., in U.S. Pat. No. 4,391,690, constructed a dual gas monitoring sensor device. Different and separate sensor cells are described: an SO.sub.2 cell having a K.sub.2 SO.sub.4 solid electrolyte, which is fed a SO.sub.2 reference gas stream, and is also connected to a source of O.sub.2 ; and an O.sub.2 cell having an oxygen ion conductive solid electrolyte, which is fed an air reference gas stream.
In the Lin et al. patent, each of the two different and separate cells have their own EMF measuring electrical circuit, to separately monitor the O.sub.2 partial pressure and the SO.sub.2 concentration of the monitored gas environment. If the O.sub.2 concentration of the monitored gas environment is shown to change by the O.sub.2 cell circuitry, the source of O.sub.2 connected to the SO.sub.2 cell could be turned on or adjusted, to establish a correct O.sub.2 balance at the SO.sub.2 cell electrodes. Another embodiment utilizes MgSO.sub.4 as a decomposable reference source of gas for the SO.sub.2 cell. These sensor designs, however, are complicated to make and operate. Also, the use of a SO.sub.2 +O.sub.2 reference gas stream, where required, is inconvenient and expensive, since a constant supply of certified tank gas is needed.
Several instances of simplified, unitary gas sensors have been disclosed in the art. Isenberg, in U.S. Pat. No. 3,915,830, relating to O.sub.2 sensors, taught hermetically encapsulating a metal/metal oxide reference medium, such as nickel/nickel oxide, exhibiting a stable oxygen activity, within a small, stabilized zirconia solid electrolyte disc. A metal electrode is attached to the outside of the solid electrolyte and is in electronic communication with the encapsulated reference medium. Sealing other reference media, such as oxygen gas or air within the solid electrolyte is also mentioned. Inoue et al., in U.S. Pat. No. 4,399,017, taught encapsulation of an electrode within a microporous, stabilized zirconia solid electrolyte. A second electrode is attached to the outside of the solid electrolyte, and the whole covered with porous ceramic. Upon application of a DC current, migration of oxygen ions, and diffusion of oxygen gas through the microporous solid electrolyte, can establish a reference partial pressure of oxygen at the interface between the microporous solid electrolyte and the encapsulated electrode, to enable measurement of oxygen gas content in flue gas.
Pebler, in U.S. Pat. No. 4,394,240, taught triangular, combination, multisensor electrochemical cells, which form an internal cavity which contains a common internal gas forming reference. In the triangular configuration, two sides are made of stabilized zirconia, oxygen ion conductive solid electrolyte and measure partial pressure of O.sub.2, and the third side can be made of K.sub.2 SO.sub.4 solid electrolyte when the partial pressure of SO.sub.3 or SO.sub.2 gases are to be measured. Reference electrodes are disposed on the inside electrolyte walls of the triangular configuration and sensing electrodes are disposed on the outside electrolyte walls.
The measuring concept in Pebler utilizes heating a central, enclosed, MgSO.sub.4, MnSO.sub.4 or Ag.sub.2 SO.sub.4 reference material, which provides SO.sub.3 on decomposition. This reference material must be kept sealed from K.sub.2 SO.sub.4 electrolyte, because of the possible reaction of these two components at high temperatures. Each of the three cells has its own circuitry. Two cells are exposed to flue gas, and one of the zirconia cells is exposed to an environment of known oxygen partial pressure, such as air. In a somewhat similar concept, involving two contacting electrolyte portions of two different gas monitors, Poirier et al., in U.S. Pat. No. 4,295,939, teach use of a reference medium, such as MgSO.sub.4 plus MgO, which upon application of heat provides a metal oxide plus SO.sub.2 within a cavity adjacent to K.sub.2 SO.sub.4 solid electrolyte of one cell, and use of a metal/metal oxide reference medium in a cavity of a second cell enclosed in a stabilized zirconia solid electrolyte.
None of these designs provide a simple, inexpensive construction that would be effective to measure SO.sub.2, CO.sub.2 or NO.sub.2 content of flue gases. Lin et al., in U.S. Ser. No. 175,434, filed on Mar. 30, 1988, (W.E. Case No. 53,216), teach a simplified, inexpensive, unitary, self-generating reference gas sensor. There, a reference electrode is isolated from the monitored gas environment by solid electrolyte, and the solid electrolyte itself, upon the application of heat, is effective to dissociate and provide the sole source of a self-generated gas, such as SO.sub.2 +O.sub.2, CO.sub.2 +O.sub.2, or NO.sub.2 +O.sub.2 at the reference electrode. That design could measure only SO.sub.2 +O.sub.2, CO.sub.2 +O.sub.2, or NO.sub.2 +O.sub.2, so that a separate O.sub.2 sensor would have to be installed along with the Lin et al. sensor, and the O.sub.2 concentration, in terms of voltage output, would have to be compensated for electronically. What is needed is a further advanced design. It is an object of this invention to provide such a construction.